Spotlight summary: Led by many advantages such as higher robustness, lower maintenance cost, and higher beam quality at high average power, fiber lasers keep enlarging their territory into the traditional field of conventional gas and solid state lasers. In industry, these benefits are directly connected to an increase in the efficiency of laser processing, leading to a higher profit. Recently, in the printed electronics industry, a higher accuracy and precision of laser engraving on metallic surfaces for printing have become more critical as the size of printed electronics gets smaller. Thus, for a better precision of laser engraving with an efficient processing time, it is becoming increasingly popular to use ultrashort fiber lasers with high average power. This is because a smaller heat-affected zone with a shorter pulse duration significantly increases the precision of the laser processing, and a higher average power shortens the processing time. At fixed repetition rate and pulse duration, one way to increase average power is to amplify the energy of each pulse. However, as the energy of the pulse grows, the peak power of the pulse also increases, eventually leading to damage of laser system as well as nonlinear phase accumulation, which causes distortion and spectral broadening of the laser pulse. Therefore, to avoid these nonlinear effects, laser amplifiers have employed two techniques, chirped-pulse amplification and divided-pulse amplification, which reduce the peak power of the laser pulse by either increasing pulse duration or dividing a pulse into many pulses prior to amplification in the gain medium.
In this Letter, the authors propose a new type of high-power ultrafast fiber laser system, namely divided-pulse lasers, which can scale the pulse energy by a factor of 16 without using separate laser amplifiers. This is achieved by creatively implementing a divide-pulse amplification technique within the laser cavity. A brief description of the authors' laser system is as follows: they first prepare an ytterbium-doped fiber soliton laser producing soliton pulses with an energy of 0.35 nJ at 1.4 ps duration. Next, they cleverly install 4 yttrium orthovanadate crystals (YVO4) as 4 dividers and 4 combiners prior to the Yb gain fiber. This significantly reduces the peak power of each pulse before the gain fiber by dividing the incoming pulses into 17 pulses, since each crystal can produce two copies of the incoming pulse. Therefore, nonlinear phase accumulation from the gain fiber can be well controlled in this laser cavity until the peak power of each divided pulse reaches the peak power that a single pulse would have reached within the laser cavity, all this without the need of installing dividers and combiners. Consequently, the energy of the recombined output pulse can be scaled by about 16 times while keeping the same pulse duration of 1.4 ps.
In summary, the authors successfully demonstrate divided-pulse lasers, which are the combination of ultrashort fiber laser and divided-pulse amplification within the laser cavity. As the authors note, the technique used in this manuscript may be potentially useful to scale the pulse energy of many other fiber lasers using different gain media without sacrificing the quality of the laser pulses, as long as the polarizations of the divided pulses are well controlled within the gain medium.
--Taek Yong Hwang
Technical Division: Optoelectronics
ToC Category: Lasers and Laser Optics
|OCIS Codes:||(140.3510) Lasers and laser optics : Lasers, fiber|
|(140.4050) Lasers and laser optics : Mode-locked lasers|
|(320.5550) Ultrafast optics : Pulses|
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